U.S. patent number 6,321,167 [Application Number 09/176,957] was granted by the patent office on 2001-11-20 for resistance-welding power supply apparatus.
This patent grant is currently assigned to Miyachi Technos Corporation. Invention is credited to Takashi Jochi, Mikio Watanabe.
United States Patent |
6,321,167 |
Jochi , et al. |
November 20, 2001 |
Resistance-welding power supply apparatus
Abstract
Disclosed is a resistance-welding power supply apparatus
comprising a large-capacitance capacitor for storing a welding
energy in the form of electric charge, a charging unit for charging
the capacitor up to a predetermined voltage, a transistor group
electrically connected between the capacitor and a welding
electrode on one hand, and a control unit for causing a switching
action of the transistor group at a predetermined frequency during
the weld time to provide a control of a welding current I. The
charging unit includes a charging transformer and a rectifying
circuit. The control unit includes a main control unit for
providing a control of the switching action of the transistor group
by way of a drive circuit. The control unit further includes
various sensors, measuring circuits and an arithmetic circuit, for
providing a feedback control of the welding current, an
interelectrode voltage or a welding power.
Inventors: |
Jochi; Takashi (Chiba-ken,
JP), Watanabe; Mikio (Chiba-ken, JP) |
Assignee: |
Miyachi Technos Corporation
(Chiba-ken, JP)
|
Family
ID: |
18033910 |
Appl.
No.: |
09/176,957 |
Filed: |
October 22, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Oct 29, 1997 [JP] |
|
|
9-312827 |
|
Current U.S.
Class: |
702/60; 702/65;
219/110; 702/64 |
Current CPC
Class: |
B23K
11/252 (20130101) |
Current International
Class: |
B23K
11/25 (20060101); B23K 011/24 () |
Field of
Search: |
;702/60-65,79,124-126,176,177,189,FOR 103/ ;702/FOR 104/ ;702/FOR
106/ ;702/FOR 109/ ;702/FOR 134/
;219/110,76.13,130.4,130.31,130.32,130.33,113,115,130.5,130.51,130.1,130.21
;363/89,41,16,17 ;323/282,349-351 ;324/134 ;700/296-298 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wachsman; Hal
Attorney, Agent or Firm: Wenderoth, Lind & Ponack,
L.L.P.
Claims
What is claimed is:
1. A resistance-welding power supply apparatus for resistance
welding materials to be welded by causing a welding current to flow
through a pair of welding electrodes which are in press contact
with the materials to be welded, said resistance-welding power
supply apparatus comprising:
a capacitor having one end electrically connected to one electrode
of the pair of welding electrodes for storing a welding energy in
the form of an electric charge;
charging means for charging said capacitor;
a transistor having a first terminal electrically connected to the
other end of said capacitor and having a second terminal
electrically connected to the other electrode of the pair of
welding electrodes; and
control means electrically connected to a control terminal of said
transistor for providing a switching control of said transistor at
a predetermined frequency during a preset weld time.
2. A resistance-welding power supply apparatus according to claim
1, wherein:
said transistor is in the form of a plurality of transistors which
are connected in parallel with one another.
3. A resistance-welding power supply apparatus according to claim
2, wherein:
said charging means comprises:
a transformer for stepping down an alternating-current (AC) power
supply voltage having a commercial frequency to a predetermined
voltage; and
a rectifying circuit for rectifying said AC power supply voltage
from said transformer into a direct-current (DC) voltage as an
output of said rectifying circuit.
4. A resistance-welding power supply apparatus according to claim
1, wherein:
said charging means comprises:
a transformer for stepping down an alternating-current (AC) power
supply voltage having a commercial frequency to a predetermined
voltage; and
a rectifying circuit for rectifying said AC power supply voltage
from said transformer into a direct-current (DC) voltage as an
output of said rectifying circuit.
5. A resistance-welding power supply apparatus according to claim
1, wherein:
said control means comprises:
current measuring means for measuring the welding current during
the preset weld time;
current setting means for providing a desired welding current set
value;
current comparing means for comparing a welding current measurement
value from said current measuring means with the welding current
set value to obtain a comparison error for each unit cycle of the
predetermined frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
6. A resistance-welding power supply apparatus according to claim
5, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
7. A resistance-welding power supply apparatus according to claim
1, wherein:
said control means comprises:
voltage measuring means for measuring a voltage between the pair of
welding electrodes during the preset weld time;
voltage setting means for providing a desired interelectrode
voltage set value;
voltage comparing means for comparing an interelectrode voltage
measurement value from said voltage measuring means with the
interelectrode voltage set value to obtain a comparison error for
each unit cycle of the predetermined frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
8. A resistance-welding power supply apparatus according to claim
7, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
9. A resistance-welding power supply apparatus according to claim
1, wherein:
said control means comprises:
current measuring means for measuring the welding current during
the preset weld time;
voltage measuring means for measuring a voltage between said pair
of welding electrodes during the preset weld time;
power computing means for computing a welding power fed to in
between the pair of welding electrodes on the basis of a welding
current measurement value from said current measuring means and an
interelectrode voltage measurement value from said voltage
measuring means;
power setting means for providing a desired welding power set
value;
power comparing means for comparing a welding power computed value
from said power computing means with the welding power set value to
obtain a comparison error for each unit cycle of the predetermined
frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
10. A resistance-welding power supply apparatus according to claim
9, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
11. A resistance-welding power supply apparatus for resistance
welding materials to be welded by causing a welding current to flow
through a pair of welding electrodes which are in press contact
with the materials to be welded, said resistance-welding power
supply apparatus comprising:
a capacitor for storing a welding energy in the form of an electric
charge;
charging means for charging said capacitor;
a first transistor having a first terminal electrically connected
to one electrode of said pair of welding electrodes and having a
second terminal electrically connected to one end of said
capacitor;
a second transistor having a first terminal electrically connected
to the other end of said capacitor and having a second terminal
electrically connected to the other electrode of the pair of
welding electrodes;
a third transistor having a first terminal electrically connected
to the other electrode of said pair of welding electrodes and
having a second terminal electrically connected to the one end of
said capacitor;
a fourth transistor having a first terminal electrically connected
to the other end of said capacitor and having a second terminal
electrically connected to the one electrode of said pair of welding
electrodes;
control means electrically connected to a control terminal of each
of said first to fourth transistors for providing a switching
control of said first and second transistors or of said third and
fourth transistors at a predetermined frequency during a preset
weld time.
12. A resistance-welding power supply apparatus according to claim
11, wherein:
said first to fourth transistors each is in the form of a plurality
of transistors which are connected in parallel with one
another.
13. A resistance-welding power supply apparatus according to claim
12, wherein:
said charging means comprises:
a transformer for stepping down an alternating-current (AC) power
supply voltage having a commercial frequency to a predetermined
voltage; and
a rectifying circuit for rectifying said AC power supply voltage
from said transformer into a direct-current (DC) voltage as an
output of said rectifying circuit.
14. A resistance-welding power supply apparatus according to claim
12, wherein:
said control means provide a switching control of said first and
second transistors, with said third and fourth transistors
remaining off, during a first preset weld time, the first preset
weld time and a second preset weld time constituting a weld time
for one resistance welding; and
said control means provide a switching control of said third and
fourth transistors, with said first and second transistors
remaining off, during said second preset weld time.
15. A resistance-welding power supply apparatus according to claim
11, wherein:
said charging means comprises:
a transformer for stepping down an alternating-current (AC) power
supply voltage having a commercial frequency to a predetermined
voltage; and
a rectifying circuit for rectifying said AC power supply voltage
from said transformer into a direct-current (DC) voltage as an
output of said rectifying circuit.
16. A resistance-welding power supply apparatus according to claim
11, wherein:
said control means provide a switching control of said first and
second transistors, with said third and fourth transistors
remaining off, during a first preset weld time, the first preset
weld time and a second preset weld time constituting a weld time
for one resistance welding; and
said control means provide a switching control of said third and
fourth transistors, with said first and second transistors
remaining off, during the said second preset weld time.
17. A resistance-welding power supply apparatus according to claim
11, wherein:
said control means comprises:
current measuring means for measuring the welding current during
the preset weld time;
current setting means for providing a desired welding current set
value;
current comparing means for comparing a welding current measurement
value from said current measuring means with the welding current
set value to obtain a comparison error for each unit cycle of the
predetermined frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
18. A resistance-welding power supply apparatus according to claim
17, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
19. A resistance-welding power supply apparatus according to claim
11, wherein:
said control means comprises:
voltage measuring means for measuring a voltage between the pair of
welding electrodes during the preset weld time;
voltage setting means for providing a desired interelectrode
voltage set value;
voltage comparing means for comparing an interelectrode voltage
measurement value from said voltage measuring means with the
interelectrode voltage set value to obtain a comparison error for
each unit cycle of the predetermined frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
20. A resistance-welding power supply apparatus according to claim
19, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
21. A resistance-welding power supply apparatus according to claim
11, wherein:
said control means comprises:
current measuring means for measuring the welding current during
the preset weld time;
voltage measuring means for measuring a voltage between the pair of
welding electrodes during the preset weld time;
power computing means for computing a welding power fed to in
between the pair of welding electrodes on the basis of a welding
current measurement value from said current measuring means and an
interelectrode voltage measurement value from said voltage
measuring means;
power setting means for providing a desired welding power set
value;
power comparing means for comparing a welding power computed value
from said power computing means with the welding power set value to
obtain a comparison error for each unit cycle of the predetermined
frequency; and
pulse width control means for controlling a pulse width for
switching-on time in the next unit cycle in response to the
comparison error.
22. A resistance-welding power supply apparatus according to claim
21, wherein:
said control means issue one or several low-frequency pulses having
a large pulse width immediately after the start of the preset weld
time after which a switching to a predetermined high frequency is
made to provide the pulse width control.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a power supply apparatus
for resistance welding, and more particularly to a power supply
apparatus of a type using transistors for the control of a welding
current.
2. Description of the Related Art
In order to provide a control of a welding current discharged from
a capacitor for storing a welding energy, a transistor-type
resistance welding power supply apparatus makes substantial use of
only transistors intervening between the capacitor and welding
electrodes without interposing a welding transformer therebetween.
This system is considered to be advantageous for use in fine spot
welding due to rapid build-up properties of the welding current and
to an arbitrary provision of upslope or downslope waveform
control.
Such a power supply apparatus ordinarily provides a feedback
constant power control so as to allow the amount of welding heat to
be kept at a certain value in spite of a possible change in the
welding current or a voltage across welding electrodes during the
weld time. For the provision of this feedback control, a shunt
resistor is disposed in a power supply circuit to detect the
welding current, with voltage sensor leads connected to the welding
electrodes or their support members to detect a voltage between the
welding electrodes so that the welding power is computed from the
welding current detection value and the voltage detection value.
Note that the transistor is in the form of a transistor group
including several tens of transistors which are connected in
parallel to one another, each transistor being a power transistor
such as an FET or IGBT which withstands a large amount of
current.
Such a conventional resistance welding power supply apparatus
causes the transistor group to act as a kind of variable resistor
between the capacitor and the welding electrodes so that provision
is made of a control of the welding current so as to allow the
welding power to coincide with the set value, for example, as
described above. For this reason, there arises a problem that the
transistor group may consume as the resistance heat in vain most of
the energy discharged (fed) from the capacitor, resulting in a poor
power supply efficiency. Furthermore, such a poor efficiency may
prevent an acquisition of a large amount of current or elongated
weld time.
SUMMARY OF THE INVENTION
The present invention was conceived in view of the above problems.
It is therefore the object of the present invention to provide a
capacitor-type resistance welding power supply apparatus ensuring
an improved power supply efficiency as well as an acquisition of a
large amount of welding current and longer weld time.
In order to achieve the above object, according to a first aspect
of the present invention, there is provided a resistance-welding
power supply apparatus for resistance welding materials to be
welded by causing a welding current to flow through a pair of
welding electrodes which are in press contact with said materials
to be welded, the resistance-welding power supply apparatus
comprising a capacitor having one end electrically connected to one
of the pair of welding electrodes, for storing a welding energy in
the form of electric charge; charging means for charging the
capacitor; a transistor having a first terminal electrically
connected to the other end of the capacitor and having a second
terminal electrically connected to the other of the pair of welding
electrode; and control means electrically connected to a control
terminal of the transistor, for providing a switching control of
the transistor at a predetermined frequency during a preset weld
time.
In the above configuration, the transistor may comprise a plurality
of transistors which are connected in parallel with one
another.
According to a second aspect of the present invention, there is
provided a resistance-welding power supply apparatus for resistance
welding materials to be welded by causing a welding current to flow
through a pair of welding electrodes which are in press contact
with the materials to be welded, the resistance-welding power
supply apparatus comprising a capacitor for storing a welding
energy in the form of electric charge; charging means for charging
the capacitor; a first transistor having a first terminal
electrically connected to one of the pair of welding electrodes and
having a second terminal electrically connected to one end of the
capacitor; a second transistor having a first terminal electrically
connected to the other end of the capacitor and having a second
terminal electrically connected to the other of the pair of welding
electrodes; a third transistor having a first terminal electrically
connected to the other of the pair of welding electrodes and having
a second terminal electrically connected to the one end of the
capacitor; a fourth transistor having a first terminal electrically
connected to the other end of the capacitor and having a second
terminal electrically connected to the one of the pair of welding
electrodes; control means electrically connected to the first to
fourth transistors via their respective control terminals, for
providing a switching control of the first and second transistors
or of the third and fourth transistors at a predetermined frequency
during a preset weld time.
In the above configuration, the first to fourth transistors may
each comprise a plurality of transistors which are connected in
parallel with one another.
Preferably, the charging means include a transformer for stepping
down an AC power supply voltage having a commercial frequency to a
predetermined voltage; and a rectifying circuit for rectifying the
AC voltage from the transformer into a DC voltage for the
output.
Preferably, the control means provide a switching control of the
first and second transistors, with the third and fourth transistors
remaining off, during a first weld time constituting each weld time
together with a second weld time for resistance welding, and the
control means provide a switching control of the third and fourth
transistors, with the first and second transistors remaining off,
during a second weld time constituting each weld time together with
a first weld time for resistance-welding.
The control means may include current measuring means for measuring
the welding current during the weld time; current setting means for
providing a desired welding current set value; current comparing
means for comparing a welding current measurement value from the
current measuring means with the welding current set value to
obtain a comparison error for each unit cycle of the frequency; and
pulse width control means for providing a control of a pulse width
for a switching-on time in the next unit cycle in response to the
comparison error.
The control means may include voltage measuring means for measuring
a voltage between the pair of welding electrodes during the weld
time; voltage setting means for providing a desired interelectrode
voltage set value; voltage comparing means for comparing an
interelectrode voltage measurement value from the voltage measuring
means with the interelectrode voltage set value to obtain a
comparison error for each unit cycle of the frequency; and pulse
width control means for providing a control of a pulse width for a
switching-on time in the next unit cycle in response to the
comparison error.
The control means may include current measuring means for measuring
the welding current during the weld time; voltage measuring means
for measuring a voltage between the pair of welding electrodes
during the weld time; power computing means for figuring out a
welding power fed in between the pair of welding electrodes, on the
basis of a welding current measurement value from the current
measuring means and of an interelectrode voltage measurement value
from the voltage measuring means; power setting means for providing
a desired welding power set value; power comparing means for
comparing a welding power computed value from the power computing
means with the welding power set value to obtain a comparison error
for each unit cycle of the frequency; and pulse width control means
for providing a control of a pulse width for a switching-on time in
the next unit cycle in response to the comparison error.
Preferably, the control means issue one or several low-frequency
pulses having a large pulse width immediately after the start of
the weld time, after which a switching to a predetermined high
frequency is made to provide the pulse width control.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, aspects, features, and advantages of
the present invention will become more apparent from the following
detailed description when reference is made to the accompanying
drawings, in which:
FIG. 1 is a block diagram showing a configuration of a
resistance-welding power supply apparatus in accordance with an
embodiment of the present invention;
FIG. 2 is a block diagram showing a functional configuration of a
main control unit of the power supply apparatus according to the
present invention;
FIG. 3 is a waveform diagram showing an example of operations of
the power supply apparatus according to the present invention;
FIGS. 4A and 4B are waveform diagrams showing an example of a
welding current build-up when almost a maximum pulse width is
applied in accordance with the present invention;
FIGS. 5A and 5B are waveform diagrams showing another example of
the welding current build-up when a pulse width exceeding the
maximum pulse width is applied in accordance with the present
invention;
FIG. 6 is a block diagram showing a configuration of the
resistance-welding power supply apparatus in accordance with
another embodiment of the present invention;
FIGS. 7A to 7D are fragmentary sectional views showing operations
at welding parts in case the power supply apparatus of FIG. 6 is
applied to a series welding;
FIG. 8 is a waveform diagram showing control pulses and a welding
current in case the power supply apparatus of FIG. 6 is applied to
the series welding; and
FIG. 9 is a waveform diagram (a nugget compensation waveform)
showing other control pulses and the welding current in case the
power supply apparatus of FIG. 6 is applied to the series
welding.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will now be
described with reference to the accompanying drawings.
FIG. 1 illustrates a configuration of a resistance-welding power
supply apparatus in accordance with an embodiment of the present
invention.
The resistance-welding power supply apparatus comprises a
large-capacitance capacitor 20 for storing welding energy in the
form of electric charge, a charging unit 18 for charging the
capacitor 20 up to a predetermined voltage, a plurality of
transistors (a transistor group) 22 electrically connected in
parallel with one another between the capacitor 20 and a welding
electrode 24 on one hand, and a control unit 30 for allowing a
switching action of the transistor group 22 at a predetermined
frequency during the weld time to thereby provide a control of a
welding current I.
The charging unit 18 includes a charging transformer 14 and a
rectifying circuit 16. The charging transformer 14 has a primary
coil connected via a main power supply switch 12 to an AC power
supply 10 (e.g., 200V) with a commercial frequency, and a secondary
coil for providing as its output a voltage which has been stepped
down to, e.g., 30V. The rectifying circuit 16 is in the form of a
single-phase hybrid bridge rectifier consisting of two thyristors S
and two diodes D which are bridge-connected to each other. The
rectifying circuit 16 rectifies an AC voltage from the charging
transformer 14 into a DC voltage to charge the capacitor 20 up to a
predetermined voltage, e.g., 24V. Note that the thyristors S are
controlled so as to be fired in synchronism with a cycle of the
commercial AC power supply 10 by means of a firing circuit for
charging (not shown).
The capacitor 20 can be a single or a plurality of low-voltage
large-capacitance type capacitors which are connected in parallel
with one another, having a capacitance of the order of, e.g., 1.2
F.
The transistor group 22 includes a plurality of, e.g., 25 parallel
connected high-speed transistors TR.sub.1, TR.sub.2, . . . TR.sub.n
such as FETs (field-effect transistors) or IGBTs (insulated gate
bipolar transistors), with each transistor TR.sub.i having a first
terminal (e.g., collector or drain) electrically connected to a
terminal on one hand of the capacitor 20 and a second terminal
(emitter or source) electrically connected to the welding electrode
24 on the other. Each transistor TR.sub.i further has a control
terminal (base or gate) connected to an output terminal of a drive
circuit 44.
The welding electrode 24 and a welding electrode 26 are coupled to
a pressing mechanism (not shown) so that they are brought into
press contact with materials W1 and W2 to be welded together during
the welding operation. Note that the welding electrode 26 is
electrically connected to a terminal of the capacitor 20.
The control unit 30 includes a main control unit 32 for providing a
control of the switching action of the transistor group 22 by way
of the drive circuit 44. The control unit 30 may further include
various sensors, measuring circuits, and computing circuits for
providing a feedback control of a welding current I, an
interelectrode voltage, or a welding power.
This configuration includes a current sensor 34 in the form of,
e.g., a toroidal coil attached to a conductor through which the
welding current I flows, a current measuring circuit 36 connected
to an output terminal of the current sensor 34, a voltage measuring
circuit 40 connected via voltage sensor leads 38 to the two welding
electrodes 24 and 26, and a power computing circuit 42 connected to
output terminals of both the current measuring circuit 36 and the
voltage measuring circuit 40.
On the basis of an output signal from the current sensor 34, the
current measuring circuit 36 acquires, as a current measurement
value, and effective value or a mean value of the welding current I
for each cycle of a switching frequency, to impart each current
measurement value I.sub.m to the main control unit 32.
On the basis of an interelectrode voltage detection signal fed
through the voltage sensor leads 38, the voltage measuring circuit
40 acquires, as an interelectrode voltage measurement value, an
effective value or a mean value of an interelectrode voltage V for
each cycle of the switching frequency, to impart each voltage
measurement value V.sub.m to the main control unit 32.
Based on a current measurement value from the current measuring
circuit 36 and on an interelectrode voltage measurement value from
the voltage measuring circuit 40, the power computing circuit 42
computes a welding power for each cycle to impart each welding
power computed value P.sub.M to the main control unit 32.
It is to be appreciated that the current measuring circuit 36, the
voltage measuring circuit 40 and the power computing circuit 42 can
be either analog circuits or digital circuits. In case of using the
analog circuits, their respective output signals (measurement
values, computed value) could be converted by an analog-to-digital
converter (not shown) into digital signals, the resultant signals
being fed to the main control unit 32.
The main control unit 32 can be a microprocessor (CPU), a dedicated
logic circuit or the like and includes functions of a setting unit,
a sequence control unit, a PWM (pulse width modulation) control
unit, etc., as will be described later.
An input unit 46 consists of pointing devices such as a keyboard
and a mouse, and of a desired interface circuit. Entered through
the input unit 46 are data in the form of set values of various
conditions including weld time, pulse width initial value, welding
current, interelectrode voltage and welding power. The data entered
through the input unit 46 are held by a memory in the main control
unit 32.
FIG. 2 is a block diagram showing a functional configuration of the
main control unit 32.
From functional aspects, the main control unit 32 comprises a
setting unit 50 for imparting to the respective units the set
values (data) of various conditions entered through the input unit
46, a sequence control unit 52 for providing a control of the weld
time, a comparing unit 54 for comparing measurement values or a
computed value with the set values to obtain comparison errors for
feedback control, a selector unit 62 for selecting one of the
various comparison errors derived from the comparing unit 54, and a
PWM pulse generating unit 64 for determining a control pulse width
in the next cycle in response to the comparison error selected by
the selector unit 62 to generate a control pulse CP having this
pulse width.
The comparing unit 54 includes a current comparing unit 56 for
comparing the welding current measurement value I.sub.M from the
current measuring circuit 36 with a welding current set value
I.sub.S to obtain a current comparison error ER.sub.I, a voltage
comparing unit 58 for comparing the interelectrode voltage
measurement value V.sub.M from the voltage measuring circuit 40
with an interelectrode voltage set value V.sub.S to obtain a
voltage comparison error ER.sub.V, and a power comparing unit 60
for comparing the welding power computed value P.sub.M from the
power computing circuit 42 with a welding power set value P.sub.S
to obtain a power comparison error ER.sub.P.
The set values I.sub.S, V.sub.S and P.sub.S fed by the setting unit
50 may be desired fixed values or may be waveform values which vary
arbitrarily with time in the form of desired waveforms.
If the selector unit 62 selects the current comparison error
ER.sub.I from the current comparing unit 56, the PWM pulse
generating unit 64 generates a control pulse CP for causing the
welding current I to coincide with the welding current set value
I.sub.S. If a selection is made of the voltage comparison error
ER.sub.V from the voltage comparing unit 58, generated is a control
pulse CP for causing the interelectrode voltage V to coincide with
the interelectrode voltage set value V.sub.S. If the power
comparison error ER.sub.P from the power comparing unit 60 is
selected, there occurs a generation of a control pulse CP for
causing the welding power P to coincide with the welding power set
value P.sub.S.
The PWM pulse generating unit 64 not merely accepts the comparison
error signals ER from the selector unit 62 but also accepts a clock
CK having a high frequency, e.g., 20 kHz defining the frequency
(switching frequency) f of the control pulse CP from a clock
circuit (not shown). During the weld time, the PWM pulse generating
unit 64 operates under the control of the sequence control unit
52.
FIG. 3 illustrates by way of example an operation of this
resistance welding control unit. In FIG. 3, weld time TE.sub.E1,
TE.sub.E2, T.sub.E3, etc., is defined as a time during which the
sequence control unit 52 allows an action of the PWM pulse
generating unit 64 in compliance with a weld time set value
registered in the setting unit 50. Halt time T.sub.H1, T.sub.H2,
etc., is a time during which the sequence control unit 52 halts the
action of the PWM pulse generating unit 64 in compliance with a
halt time set value registered in the setting unit 50.
During each weld time T.sub.E, the PWM pulse generating unit 64
issues a control pulse CP at the switching frequency f (20 kHz), in
response to which control pulse CP the drive circuit 44 provides a
switching drive of all the transistors TR.sub.1, TR.sub.2, . . .
TR.sub.n of the transistor group 22 simultaneously, that is, at the
same timing.
Such a switching action of all the transistors TR.sub.1, TR.sub.2,
. . . TR.sub.n of the transistor group 22 allows the capacitor(s)
20 to release the electric charge via the transistor group 22 to
the welding electrodes 24 and 26 and to the materials W.sub.1 and
W.sub.2 to be welded together, allowing a flow of direct welding
current I.
At that time, each transistor TR.sub.i of the transistor group 22
repeats its on-off action at a high frequency, with its on-action
causing a conduction in a saturated state, so that a less
resistance heat is gererated, resulting in a reduced power loss.
This allows energy released (discharged) from the capacitor(s) 20
to be effectively delivered to the materials W.sub.1 and W.sub.2 to
be welded together, making it possible to provide a flow of a large
quantity of welding current I as well as to elongate each weld time
T.sub.E.
Furthermore, the PWM control of a feedback method is employed for
providing a variable control of the pulse width of the control
pulse CP, so that it is possible to control the waveforms of the
welding current I, interelectrode voltage V and welding power P so
as to have arbitrary patterns.
As is apparent from FIGS. 4A and 4B, the build-up time of the
welding current I can be reduced by setting to the maximum pulse
width or its vicinity the pulse width (pulse width initial value)
of the control pulse CP first output immediately after the start of
the weld time T.sub.E upon the build-up of the welding current
I.
Alternatively, as shown in FIGS. 5A and 5B, the PWM control may be
provided through a changeover to the switching frequency f after
the output of one or several pulses having a pulse width ts
exceeding the maximum pulse width (tc) of the switching frequency f
immediately after the start of the weld time. This method achieves
a further reduction of the build-up time and is advantageous when
using a welding current I set to a large current value. using a
welding current I set to a large current value.
In this manner, this resistance-welding control unit provides
excellent build-up characteristics and a waveform control as well
as a high power supply efficiency, with the securement of an
increased welding current and elongated weld time, whereby it is
applicable to various metal materials (materials to be welded) in
fine spot welding and ensures a reliable weld quality.
FIG. 6 illustrates a configuration of a resistance-welding power
supply apparatus in accordance with another embodiment of the
present invention. In the diagram, the same reference numerals are
imparted to parts having the same configurations and functions as
those in the power supply apparatus of the above first
embodiment.
This resistance-welding power supply apparatus comprises four sets
of transistors, that is, first to forth transistor groups 22A',
22A, 22B' and 22B.
The first transistor group 22A' includes a predetermined number of,
e.g., 25 high-speed transistors TR.sub.Al' to TR.sub.An' such as
FETs or IGBTs which are connected in parallel with one another,
with each transistor TR.sub.Ai' having a first terminal
electrically connected to the welding electrode 26 and a second
terminal electrically connected to a terminal of the capacitor 20.
Each transistor TR.sub.Ai' further has a control terminal connected
to a first output terminal of a drive circuit 44A.
The second transistor group 22A includes a predetermined number of,
e.g., 25 high-speed transistors TR.sub.Al, to TR.sub.An such as
FETs or IGBTs which are connected in parallel with one another,
with each transistor TR.sub.Ai having a first terminal electrically
connected to a terminal of the capacitor 20 and a second terminal
electrically connected to the welding electrode 24. Each transistor
TR.sub.Ai further has a control terminal connected to a second
output terminal of the drive circuit 44A.
The third transistor group 22B' includes a predetermined number of,
e.g., 25 high-speed transistors TR.sub.B1' to TR.sub.Bn' such as
FETs or IGBTs which are connected in parallel with one another,
with each transistor TR.sub.Bi' having a first terminal
electrically connected to the welding electrode 24 on the other and
a second terminal electrically connected to the terminal of the
capacitor 20. Each transistor TR.sub.Ai' further has a control
terminal connected to a first output terminal of a drive circuit
44B.
The fourth transistor group 22B includes a predetermined number of,
e.g., 25 high-speed transistors TR.sub.B1 to TR.sub.Bn such as FETs
or IGBTs which are connected in parallel with one another, with
each transistor TR.sub.Bi having a first terminal electrically
connected to the terminal of the capacitor 20 and a second terminal
electrically connected to the welding electrode 26. Each transistor
TR.sub.Bi further has a control terminal connected to a second
output terminal of the drive circuit 44B.
In this resistance-welding power supply apparatus, the main control
unit 32 provides a control such that a selective switching action
is conferred on the first and second transistor groups (22A', 22A)
and the third and fourth transistor groups (22B', 22B) by way of
the drive circuits 44A and 44B, respectively.
When the first and second transistor groups (22A', 22A) undergo the
selective switching action, a normal direction welding current
I.sub.A flows through the welding electrodes 24, 26 and the
materials W.sub.1, W.sub.2 to be welded together. This mode
corresponds to the power supply apparatus in accordance with the
first embodiment described above.
On the contrary, the selective switching action of the third and
fourth transistor groups (22B', 22B) allows a reverse direction
welding current I.sub.B to flow therethrough.
This resistance-welding power supply apparatus is conveniently
applied to two-point simultaneous joint-type resistance welding
(series welding). Referring then to FIGS. 7A to 7D, FIGS. 8 and 9,
description is made of its operation in case of application to the
series welding.
In case a seam welding is done for the materials (W.sub.1, W.sub.2)
to be welded together as shown in FIG. 7, the input unit 46 and the
main control unit 32 divide the weld time for each resistance
welding into first weld time T.sub.A and second weld time
T.sub.B.
Then, as shown in FIG. 8, during the first weld time T.sub.A, only
the first and second transistor groups (22A' and 22A) perform a
continuous switching action under the PWM control, whereas during
the second weld time T.sub.B, only the third and fourth transistor
groups (22B' and 22B) perform the continuous switching action under
the PWM control. Preferably, a constant current control is usually
provided to keep the welding current I at a certain value.
Accordingly, as shown in FIG. 8, the first weld time T.sub.A allows
a flow of a positive welding current I.sub.A having substantially a
trapezoidal current waveform. In this case, as shown in FIG. 7A,
the welding current I.sub.A forms a current path which extends from
the welding electrode 24 through the material W.sub.1 to be welded,
a first welding point P.sub.a, the material W.sub.2 to be welded, a
second welding point P.sub.b and again the material W.sub.1 to be
welded to the welding electrode 26. In other words, at the first
welding point P.sub.a the welding current I.sub.A flows from the
material W.sub.1 to be welded toward the material W.sub.2 to be
welded, whereas at the second welding point P.sub.b the welding
current I.sub.A flows from the material W.sub.2 to be welded toward
the material W.sub.1 to be welded. As a result of this, for
instance, the first welding point P.sub.a is subjected to a Peltier
effect absorbing heat, whereas the second welding point p.sub.b is
subjected to a Peltier effect generating heat.
Therefore, during the first weld time T.sub.A, a nugget N.sub.b at
the second welding point P.sub.b grows at a higher growth rate than
a nugget N.sub.a at the first welding point P.sub.a. In
consequence, at the end of the first weld time T.sub.A, the nugget
N.sub.b becomes larger than the nugget N.sub.a as shown in FIG. 7B.
A difference in size of the two nuggets N.sub.a and N.sub.b at this
point of time depends on the materials (W.sub.1, W.sub.2) to be
welded together, the thickness thereof, the duration of the first
weld time T.sub.A, the set current value, etc.
However, as shown in FIG. 8, during the second weld time T.sub.B,
the third and fourth transistor groups (22B', 22B) perform the
switching action so as to allow a flow of a negative welding
current I.sub.B having substantially a trapezoidal current
waveform. In this case, as shown in FIG. 7C, the welding current
I.sub.B forms a current path which extends from the welding
electrode 26 through the material W.sub.1 to be welded, a second
welding point P.sub.b, the material W.sub.2 to be welded, a first
welding point P.sub.a and again through the material W.sub.2 to be
welded to the welding electrode 24. In other words, at the first
welding point P.sub.a, the welding current I.sub.B flows from the
material W.sub.2 to be welded toward the material W.sub.1 to be
welded toward the material W.sub.2 to be welded. Thus, this time,
the first welding point P.sub.a undergoes a Peltier effect
generating heat, whereas the second welding point p.sub.b undergoes
a Peltier effect absorbing heat. Therefore, during the second weld
time T.sub.B, a nugget N.sub.a at the first welding point P.sub.a
grows larger than a nugget N.sub.b at the second welding point
P.sub.b.
In consequence, at the end of the second weld time T.sub.B, that
is, at the completion of the entire weld time, as shown in FIG. 7D,
substantially the same size is conferred on the nugget N.sub.a at
the first welding point P.sub.a and on the nugget N.sub.b at the
second welding point P.sub.b.
Typically, however, during the first weld time T.sub.A the nugget
is already formed on each welding part to some extent, lowering the
resistance value of the conductive path. This results in a
reduction of the heat generation efficiency. Therefore, the nuggets
N.sub.a and N.sub.b during the second weld time T.sub.B have their
respective growth rates lower than those of the nuggets N.sub.a and
N.sub.b during the first weld time T.sub.A, whereupon the
difference (N.sub.a >N.sub.b) in the growth rate between the two
nuggets N.sub.a and N.sub.b during the second weld time is smaller
than the difference (N.sub.a <N.sub.b) during the first weld
time T.sub.A. As a result of this, the size of the nugget N.sub.a
may not reach the size of nugget N.sub.b at the elapse of time
equal to the first weld time T.sub.A in the second weld time
T.sub.B.
In such a case, the second weld time T.sub.B is set to be longer
than the first weld time T.sub.A by an extension time T.sub.K as
shown in FIG. 9 so that the nugget N.sub.a can catch up with the
nugget N.sub.b in the extension time T.sub.K. The extension time
T.sub.K for fulfilling this catch-up condition depends on welding
conditions such as the materials (W.sub.1, W.sub.2) to be welded,
the thickness thereof, the duration of the first weld time T.sub.A,
the set current value, etc. The extension time may be given as an
empirical value based on a test welding, etc.
Consequently, at the end of the second weld time T.sub.B,
substantially the same size is securely conferred on the nugget
N.sub.a at the first welding point P.sub.a and on the nugget
N.sub.b at the second welding point p.sub.b as shown in FIG. 7D.
Thus, a substantially uniform welding strength is imparted to the
first welding point P.sub.a and the second welding point
P.sub.b.
As an alternative, such a nugget compensation may be achieved by
setting the current value of the welding current I.sub.B during the
second weld time T.sub.B to be larger than the current value of the
welding current I.sub.A during the first weld time T.sub.A.
This alternative resistance welding control unit also provides
excellent build-up characteristics and a waveform control as well
as a high power supply efficiency, with securement of an increased
welding current and elongated weld time, whereby it is applicable
to various metal materials in the above series welding or other
precision small-sized resistance welding and ensures a reliable
welding finish.
Although the above embodiment uses the single-phase commercial AC
power supply 10, the configuration of the charging unit 18 could be
modified so as to allow an input of a three-phase commercial AC
voltage. The charging unit 18 may be provided with a voltage
detecting circuit for detecting a terminal voltage (charging
voltage) at the capacitor 20 and with a charging control unit for
comparing a voltage detection value with a set voltage value to
provide a control of charging of the capacitor 20.
Furthermore, a single transistor having a large current capacity
may be employed in place of each transistor group 22A, 22A', 22B
and 22B, including a plurality of transistors which are connected
in parallel with one another.
The configuration of the control unit 30 in the above embodiment
could also be variously modified. For example, the current sensor
34 could be a shunt resistor. Various methods and circuits are
available for the measurement and computations of the current,
voltage, power, etc. The functions of the main control unit 32
could also be variously modified from its hardware and software
aspects.
According to the resistance-welding power supply apparatus of the
present invention, as set forth hereinabove, the transistors are
interposed between the capacitors and the welding electrodes so
that the capacitors are subjected to a switching control at a
predetermined frequency during the weld time to provide a control
of the welding current or welding power, thereby making it possible
to reduce the excess power consumption of the transistors and
accordingly improve the power supply efficiency, to secure an
increased welding current and elongated weld time.
While illustrative and presently preferred embodiments of the
present invention have been described in detail herein, it is to be
appreciated that the inventive concepts may be otherwise variously
embodied and employed and that the appended claims are intended to
be construed to include such variations except insofar as limited
by the prior art.
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